1. Field of the Invention
The present invention relates to a color video printer.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a block diagram of a conventional video printer. As shown in this figure, the conventional color video printer comprises a luminance signal/color signal (Y/C) separator 1 for separating a luminance signal Y and a color signal C from a video signal inputted therein, a decoder 2 for detecting red (R), green (G) and blue (B) signals and vertical and horizontal synchronizing signals V. and H. from the color signal C from the Y/C separator 1, an analog/digital (A/D) converter 3 for converting the separated R, G and B signals from the decoder 2 into digital signals, a video memory 4 responsive to a memory control signal CS1 for writing the digital R, G and B signals from the A/D converter 3 therein in the unit of frame and reading the stored R, G and B signals therefrom, a memory controller 5 for outputting the memory control signal CS1 to the video memory 4 for the read and write operations of the video memory 4 according to a user's selection and a switching control signal CS2, a digital/analog (D/A) converter 6 for converting the digital R, G and B signals read from the video memory 4 into analog signals, a tone control let 7 for converting the digital R, G and B signals read from the video memory 4 into print three-color signals or magenta (m), cyan (c) and yellow (y) signals, a switch 8 responsive to the switching control signal CS2 from the memory controller 5 for transferring the R, G and B signals from the video memory 4 to the D/A converter 6 and the tone controller 7 in the read operation or transferring the digital R, G and B signals from the A/D converter 3 to the video memory 4 in the write operation, an encoder 9 for converting the analog R, G and B signals from the D/A converter 6 into the luminance and color signals Y and C, a Y/C mixer 10 for mixing the luminance and color signals Y and C from the encoder 9 and outputting the mixed signal to a display unit (not shown) such as a monitor for displaying a desired video based on the mixed signal, a gamma correction circuit 11 for correcting distortions of the m, y and c signals from the tone controller 7, a thermal printing head 12 for printing the desired video based on output signals from the gamma correction circuit 11, and a video adjustment circuit 13 for adjusting levels of the analog R, G and B signals from the D/A converter 6 to adjust brightness, tint and contrast of the video to be displayed.
The operation of the color video printer with the construction as mentioned above with reference to FIG. 1 will hereinafter be described.
Upon input of a video signal, the Y/C separator 1 separates the luminance signal Y and the color signal C from the inputted video signal and the decoder 2 detects the R, G and B signals and the vertical and horizontal synchronizing signals V. and H. from the color signal C from the Y/C separator 1. The R, G and B signals as video three-color signals from the decoder 2 are converted by the A/D converter 3 into the digital R, G and B signals, which are then applied to the video memory 4 through the switch 8 in response to the switching control signal CS2 from the memory controller 5. The video memory 4 writes the digital R, G and B signals from the A/D converter 3 therein in the unit of frame in response to the memory control signal CS1 for the write operation, applied from the memory controller 5. At this time, upon receiving the vertical and horizontal synchronizing signals V. and H. from the decoder 2, the memory controller 5 recognizes that the video signal has been inputted and thus outputs the memory control signal CS1 for the write operation to the video memory 4. The memory controller 5 also outputs the switching control signal CS2 to the switch 8 to connect the output of the A/D converter 3 to the input of the video memory 4.
On the contrary, upon receiving no vertical and horizontal synchronizing signals V. and H. from the decoder 2, the memory controller 5 recognizes that no video signal has been inputted and thus outputs the memory control signal CS1 for the read operation to the video memory 4. The memory controller 5 also outputs the switching control signal CS2 to the switch 8 to connect the output of the video memory 4 to the input of the D/A converter 6 and the input of the tone controller 7.
Accordingly, the digital R, G and B signals stored in the unit of frame in the video memory 4 are read by the memory control signal CS1 for the read operation from the memory controller 5 and are applied to the D/A converter 6 and the tone controller 7 through the switch 8. The switch 8 acts to connect the output of the video memory 4 to the input of the tone controller 7 and the input of the D/A converter 6 in response to the switching control signal CS2 from the memory controller 5.
In the tone controller 7, the digital R, G and B signals from the video memory 4 are converted into the print three-color signals or the magenta (m), cyan (c) and yellow (y) signals, the distortions of which are corrected in the gamma correction circuit 11. The thermal printing head 12 prints the desired video based on the distortion-corrected m, c and y signals from the gamma correction circuit 11.
On the other hand, in the D/A converter 6, the digital R, G and B signals from the video memory 4 are converted into the analog R, G and B signals, which are then converted into the luminance and color signals Y and C in the encoder 9. The Y/C mixer 10 mixes the luminance and color signals Y and C from the encoder 9 and outputs the mixed signal to the display unit such as the monitor.
The video adjustment circuit 13, as shown in FIG. 1, has a variable resistor VR1 for adjusting the levels of the analog R, G and B signals from the D/A converter 6, being applied to the encoder 9, so as to optimize the states (i.e., brightness, tint and contrast) of the video to be displayed.
However, the conventional color video printer has a disadvantage, in that a black color appears on the whole surface of a print paper in the print operation or on the whole surface of a screen in the display operation since any signal is not stored in the video memory 4 when no video signal has been inputted or at the initial state. This results in a bad view. Also, if unnecessary data is stored in the video memory 4 in the case where no video signal has been inputted, an unrelated video is printed or displayed. Moreover, since the color states of the video cannot be tested at the initial state before display through the display unit, an accurate color adjustment for the video is difficult in the display operation. Furthermore, because the color states of the video cannot be tested at the initial state before print through the thermal printing head, an accurate color adjustment for the video is difficult in the print operation.